The present invention relates generally to implantable cardiac pacemakers responsive or adaptive to patient exercise, as detected by movement or activity of the patient, to generate an appropriate pacing rate. More particularly, the invention pertains to a rate-adaptive pacemaker which develops an appropriate rate response to a specific type of exercise by the patient based on morphology of the signal generated by the activity sensor.
Since its introduction into clinical practice, rate-adaptive pacing has gained widespread application. Currently, more than 50% of all pacemakers implanted worldwide employ means for varying pacing rate based on data provided by one or more sensors.
The aim of rate-adaptive pacemakers is to pace the heart with a variable rate that matches the metabolic needs of the body, according to the state of rest or exercise of the patient. An adequate rate adaptation is mandatory to permit full physical capability of the patient. German Patent No. DE 34 19 439 and related U.S. Pat. No. 4,688,573 disclose a rate-adaptive pacer responsive to central venous blood temperature, with algorithms defining heart rate as a function of the latter for states of rest and exercise, and a decision rule for selection of an appropriate algorithm in a given situation. Various other physiologic parameters that have been proposed for detection and use in rate-adaptive pacing include blood oxygen saturation, respiration, and chest impedance (minute ventilation). These intrinsic parameters respond relatively slowly to changes in level of patient exercise, with concomitant delay in change in pacing rate. Moreover, the parameter sensors are expensive precision devices typically requiring complex surgical implant procedures.
To achieve a more physiologic, rapid response of pacing rate to patient activity with a low cost sensor and relatively uncomplicated implant procedure, the technology has moved to the use of pure activity sensors designed to detect movement or exercise of the patient directly, rather than the indirect sensing that had been principally employed. Detection of the patient's physical activity is now the most widespread principle used for rate-adaptive pacing.
The first rate-adaptive pacemaker employing activity sensing may have been disclosed in U.S. Pat. No. 4,140,132 to Dahl. A mechanoelectrical converter consisting of a weighted cantilever arm and piezoelectric crystal in the implanted pacer underwent mechanical vibrations in reponse to patient movement. These vibrations were converted by the crystal to an electrical output signal, which was used to control the variable rate pulse generator of the pacemaker. U.S. Pat. No. 4,428,378 to Anderson is to similar effect, the amplitude of the high frequency content of the converter output signal being a bandpass signal to control pacing rate.
Activity or motion sensors provide virtually immediate response to patient movement for pacing rate control, but have disadvantages of sensitivity to noise and other disturbances which contribute improperly to the pacing rate. The Anderson '378 patent proposes that the maximum sensitivity of the device be in a frequency range above 10 Hertz (Hz), which it assumed to be the resonant frequency range of the major body compartments such as thorax and abdomen, and thought to be where the maximum value of pedal impact detected by the sensor would occur.
In contrast, U.S. Pat. No. 4,926,863 to Alt teaches that the activity sensor output signal indicative of true physical exercise actually occurs in a frequency range well below 10 Hz--at about 4 Hz or less--and that this range is virtually devoid of frequencies of disturbances unrelated to exercise. This low frequency band and different baseline values established for comparison have been shown to enable rapid, accurate rate response, especially where the activity sensor is an accelerometer. The accelerometer may also be conveniently located within the pulse generator housing, with resulting cost benefit and ease of implantation.
Other improvements in rate-adaptive pacers using an accelerometer as the activity sensor are disclosed in U.S. Pat. No. 5,031,615 to Alt, which describes manufacture of the accelerometer with related processing circuitry integrated in hybrid semiconductor circuitry as a microminiature, low power mechanoelectrical transducer that filters the signal to pass only components in the band below about 4 Hz. U.S. Pat. No. 5,014,703 to Alt discloses an activity pacemaker that discriminates between patient movements attributable to true physical exercise and detected indicia arising from other causes (e.g., riding on a rough road surface) samples the output signal over successive time intervals to assess the trend of patient exercise, and to adjust pacing rate accordingly. U.S. Pat. No. 5,031,614 to Alt uses frequency and amplitude of accelerometer signal to control pacing rate, both of those components obtained from signal amplitude only, using a moving window technique. Co-pending application Ser. No. 08/279,946 to Alt discloses use of a distinct nonlinearity of the accelerometer signal to discriminate between types of patient exercise, such as walking and bicycling, despite same or similar workload, and to adjust pacing rate.
Accelerometer-based pacemakers have provided the most physiologically adequate rate adaptation with fast response time and gradually increasing pacing rate with increasing physical activity. This is especially true for walking--the most popular form of exercise for pacemaker patients. Several clinical studies have demonstrated the linear correlation between pacing rate and workload when the patient's walking speed is gradually increased.
A recent article titled "Intrinsic heart rate response as a predictor of rate-adaptive pacing benefit", E. Alt et al., Chest 1995, 107:925-30, describes the strong correlation between the body's oxygen uptake and accelerometer controlled pacing rate with walking.
Accelerometer-based pacing devices have successfully overcome many of the limitations of previous activity pacemakers, such as when patients walk on different grades, including horizontal surfaces, up and down stairs at a regular walking speed of 72 steps per minute. A recent study conducted by the applicants herein supports this finding, as reported in "Activity-controlled cardiac pacemakers during stair-walking; a comparison of accelerometer to vibration-guided devices and to sinus rate", E. Alt et al., PACE 1995). But this study also found that with higher walking speeds, such as 84, 92, 96, 108, or 120 steps per minute, which represents relatively fast walking, differences in pacing rate between walking on level surface, up stairs, and down stairs disappear. That is, under those conditions the accelerometer-based devices provide virtually the same pacing rate independent of the type of activity the patient is engaging in at the higher walking rates.
This behavior departs significantly from the normal rate behavior, in which, for example, walking upstairs yields the highest sinus rate, and walking downstairs yields the lowest sinus rate compared with the medium rate of walking on a horizontal surface, assuming the same walking speed for all three types of activity.
It is a principal aim of the present invention to provide methods and means to improve the performance of rate-adaptive pacemakers based on more accurate detection of physical activities with an accelerometer, toward more physiologic regulation.
A related object of the invention is to provide techniques for detecting and distinguishing types of patient physical exercise from one another, using the morphology of the activity signal, so as to enhance the pacing of the patient's heart at rates most closely approximating the true physiologic rate for the type of exercise involved.